Iraqi forces moved this week to re assert Baghdad’s control over the southern port city of Basra, which has been in the hands of Shiite militias – most notably, radical cleric Moqtada al-Sadr’s Mahdi Army – ever since British troops abandoned the city last year.

The offensive has also sparked attacks elsewhere by forces associated with Sadr, who’s been hiding out in Iran since he halted attacks on US and Iraqi troops seven months ago.

As we’ve noted, surprises are to be expected in war.

And while it’s still early, the current fighting could prove decisive – one way or the other.

Several signs are encouraging. As Coalition commanders are quick to point out, the Basra offensive was decided upon, planned and executed almost entirely by Iraqis – a fact that by itself speaks volumes to progress wrought by Gen. David Petraeus’ surge strategy.

One of Prime Minister Nouri al-Maliki’s chief goals is apparently control of Basra’s port; success there could win the Baghdad government much credibility.

Make no mistake, though: This is a crucial test for Iraq’s fledgling army – one that’s made no easier by Iraq’s ever-complex political realities.

The Mahdi Army, for instance, is only one of several militias that have been vying for control of Basra. Another, the Badr Brigades, is widely believed to have a strong presence on the city’s police force – and its political arm is crucial to Maliki’s coalition government.

The other enigma is Sadr, the radical thug who made a name for himself picking off American “occupiers” in the early days of the war.

It’s a disgrace he’s still alive – for his minions could sow serious chaos in Baghdad, just as Coalition forces may have al Qaeda cornered up north.

Now’s no time to go wobbly, of course. Even as Iraqi forces continue to step up in securing their country, constant American vigilance will be necessary.

Gen. Petraeus, for one, looks especially prescient in his request for a pause in American troop reductions until circumstances better reveal the true need.